In many outdoor scenarios, the wireless multipath channel exhibits multipath components whose delays are longer than the Cyclic Prefix (CP) used in multi-carrier systems such as orthogonal frequency-division multiplexing (OFDM). The aforementioned system may be referred to as “insufficient CP system”. In insufficient CP systems, the multipath components with delays longer than the CP lead to two types of interference, namely Inter-Symbol Interference (ISI) and Inter-Carrier Interference (ICI). This means that the samples of the current symbol are interfered by samples of the previous symbol (ISI), but they also exhibit self-interference, i.e. each subcarrier leaks power on the adjacent subcarriers (ICI).
Classic pilot-based (PB) channel estimators ignore ISI and ICI, which—as a result—severely affects the quality of the computed estimates. This causes an overall degradation of the receiver's performance. Data-aided iterative algorithms which sequentially refine the channel and data estimates and which cancel ISI and ICI, achieve close to optimal performance. However, such schemes suffer from high computational complexity.
In LTE OFDM systems operating over channels with maximum excess delay (MED) greater than CP duration, the receiver performance is degraded by ISI and ICI. Classic PB channel estimators neglect the interference arising in such scenarios, and therefore, compute poor channel estimates. These estimates once fed to the equalizer and decoding block, impair the BER performance of the receiver. Given their low complexity and good performance in scenarios with channel MED shorter than CP duration, PB estimators are usually preferred in the design of the LTE receiver.
It may be desirable to improve channel estimation in mobile systems of insufficient CP length without significantly increasing computational complexity.